Molecular basis of thermostability in the lysozyme from bacteriophage T4

Abstract

MOST proteins are denatured at temperatures above 50–60°C, although some enzymes, especially those from thermophilic organisms, remain active at temperatures up to 80–90 °C. The determination of the three-dimensional structure of the thermostable protease thermolysin showed that heat-stable proteins do not contain unusual structural features absent from less stable proteins1,2. Furthermore, the amino acid sequences of similar proteins from both mesophilic and thermophilic sources have been shown to be homologous, suggesting that the respective structures are similar3,4. Nevertheless, such homologous amino acid sequences also include many differences which obscure those amino acid changes actually responsible for differences in thermostability. We report here the structure of a temperature sensitive (ts) mutant of T4 phage lysozyme. This permits the first direct comparison of two protein structures in which all differences are directly related to a change in thermal stability. It is shown that, except for the replacement of a partially exposed arginine by a histidine, the three-dimensional structure of the ts lysozyme is virtually identical with that of native lysozyme.